1. Field of The Invention
The invention relates to a tachometer signal conditioner circuit which accepts a wide range of input signals and produces a digital pulse signal which indicates a shaft phase reference position.
2. Description of The Prior Art
Historically there has been a need in data acquisition and analysis systems for a data signal that could produce both a single pulse per revolution of the machine shaft and multiple pulses per revolution of the same shaft. A typical area where such a need would exist would be with regard to aircraft engines where multiple pulses could indicate the revolution speed of the engine and a single pulse could produce a phase reference signal which is required to perform calculations that will specify the proper location of the balance weights to balance the engine.
In response to the above need, some aircraft engine manufacturers have designed in a discrete reference signal (1/revolution signal), but since magnetic transducers are generally used, the signals vary in amplitude as the engine speed changes.
Other engine manufacturers measure engine speed by designing in a multiple pulse per revolution (N/revolution signal) and then making one pulse different (e.g., higher, lower, narrower, wider) to use as a phase reference signal. These N/revolution signals are also based on the use of a magnetic sensor observing the passage of teeth on a multi-toothed wheel and thus suffer the same variable voltage characteristics with engine speed changes. Although these manufacturers generally offer a means for detecting the odd pulse for the reference signal, the method offered only works on their particular wheel design.
Still other manufacturers design in the N/revolution speed signal and give no thought to its possible use as a phase reference. For these cases, it is generally possible to shave a tooth shorter to create an odd tooth reference pulse.
Finally, some engines designed do not use that 1/revolution or N/revolution pulse signal at all, but instead provide a variable voltage sinusoidal signal (a "tachometer generator sensor") at a frequency corresponding to the engine speed. As the engine speed increases, these sensors generally begin to saturate their internal circuits and thus the sine wave progressively turns into a square wave.
Thus a need exists for an electronic circuit that could accept the aforementioned diverse range of possible input signals (in addition to other signal types that might be encountered in the future) and to reliably produce a 1/revolution output reference signal. Additional requirements were that the circuit was required to be almost entirely automatic in that an operator would not be required to make complicated adjustments or settings and that the circuit needed to automatically sense whether the input was from the high or low odd-tooth signal and be able to accept a wide range of input voltages, frequencies, and signal types.